Flaky: test_concurrent_callers_share_one_transfer (test_stt_decoder.py::TestRetiredDecoderMigration) returns None for one caller #23360

Seen on Linux shard (3.12, 2) of PR #23334: https://github.com/kirodotdev/KiroCrew/actions/runs/39912714046/job/120284095276

• OS: ubuntu-latest, Python 3.12
• Failing line: FAILED test/test_stt_decoder.py::TestRetiredDecoderMigration::test_concurrent_callers_share_one_transfer - AssertionError: assert PosixPath('/tmp/pytest-of-runner/pytest-0/popen-gw2/test_concurrent_callers_share_0/ffmpeg-test') == None
• Shard result: 1 failed, 30165 passed.

Two concurrent migrate_retired_decoder() callers are expected to share one transfer and return the same path. In this run one of them returned None. That looks like a race in the module-level _migration_task the fixture resets. The test arrived with #23158. The PR it failed on does not touch the stt code. It passes locally 3/3 (pytest -n0 test/test_stt_decoder.py::TestRetiredDecoderMigration).